In order to give a steering angle to a steering wheel (front wheel in most vehicles except for special vehicles such as forklifts), as a device to reduce the force required for a driver to steer the steering wheel, a power steering device is widely used. As such a power steering device, an electric power steering device using an electric motor as an auxiliary power unit has been widely used in recent years. As to such an electric power steering device, various structures thereof are known. However, in any structure, auxiliary power of an electric motor is applied, via a reducer, to a rotation shaft that is rotated in accordance with operation of a steering wheel and which gives a steering angle to the steered wheel by being rotated. A worm reducer is typically used as the reducer. In the case of an electric power steering device using a worm reducer, a worm rotatably driven by an electric motor and a worm wheel rotating together with a rotation shaft are engaged so that the auxiliary power of an electric motor can be transmitted to the rotation shaft.
For example, Patent Document 1 discloses an electric power steering device as illustrated in FIGS. 4 and 5. A front end of a steering shaft 2 which is a rotation shaft rotated in a predetermined direction by a steering wheel 1 is rotatably supported in a housing 3, and a worm wheel 4 is fixed to this portion. A worm 6 engaged with the worm wheel 4 and rotatably driven by an electric motor 5 is formed by providing a worm tooth 8 at an axially middle portion of a worm shaft 7. A distal end of the worm shaft 7 is supported in the housing 3 by a distal end bearing 9 which is a rolling bearing such as a single row deep groove ball bearing and a base end of the worm shaft 7 is similarly supported by a proximal bearing 10 that is the same bearing as the distal end bearing.
In order to rotatably drive the worm 6 via an output shaft 11 of the electric motor 5, a spline hole 12 is formed in the base end of the worm shaft 7 such that the spline hole 12 is open at a base end surface of the worm shaft 7. Further, a distal end of the output shaft 11 is provided with a spline shaft portion 13. With the structure in which the spline shaft portion 13 is spline-locked to the spline hole 12, the output shaft 11 and the worm shaft 7 are coupled to transfer the rotational force.
In recent years, in order to improve a production rate of electric power steering devices having the structure described above, automation of assembling work for the electric power steering devices is progressing. Furthermore, automation of assembling work for worm reducers is also considered. The assembling work for a worm reducer is performed as illustrated in FIGS. 6A, 6B, and 6C and as described in Patent Document 2.
First, as illustrated in FIG. 6A, a distal end bearing 9 is inserted horizontally into a worm accommodation portion 14 having a substantially cylindrical shape with a bottom end and constituting a housing 3 through an opening of the accommodation portion 14, and is thus brought into contact with an abutting portion 15 of an inner peripheral surface of the worm accommodation portion 14. As a result, the distal end bearing 9 is internally fitted and fixed (press fitted) into a bearing receiving portion 16 provided at a deep inner portion of the worm accommodation portion 14. Thereafter, a worm wheel 4 is incorporated into a worm wheel accommodation portion 17 constituting the housing 3. Next, as sequentially illustrated in FIG. 6A and FIG. 6B, in a state in which the worm wheel 4 is engaged with a worm teeth 8, a worm 6 is inserted into the worm accommodation portion 14 while being rotated in a predetermined direction. Next, a distal end of the worm 6 is inserted into an inner portion of the distal end bearing 9. Finally, as illustrated in FIG. 6C, a base end bearing 10 is externally fitted and inserted from a base end side of the worm 6, and is internally fitted and fixed (press fitted) in a bearing receiving portion 18 formed at a middle portion of the inner peripheral surface of the worm accommodation portion 14.
Conventionally, assembling work for a worm reducer is performed according to the above-described work process. In such a work process, a process step at which the distal end bearing 9 is inserted into (internally fitted into) the bearing receiving portion 16 formed at a deep inner portion of the worm accommodation portion 14 is expected to be automated. However, there are the following problems in automating this process step.
That is, in the case of a typical power steering device, the bearing receiving portion 16 is formed at a position which is about 200 mm away from the opening of the worm accommodation portion 14, and there is merely a small margin (gap), usually about 10 μm, in a radial direction between the bearing receiving portion 16 and the outer peripheral surface of the distal end bearing 9. For this reason, when the distal end bearing 9 is inserted into the bearing receiving portion 16, an insertion position of a bearing needs to be highly precisely controlled. Furthermore, since the insertion step is performed by horizontally moving the distal end bearing 9, the insertion position is likely to be shifted downward due to the influence of gravity acting on the distal end bearing 9 and a support member that supports the distal end bearing 9. Further, due to a dimensional tolerance of the bearing receiving portion 16, the central axis of the bearing receiving portion 16 is likely to be deviated by about 0.6 mm with respect to a reference axis that lies with a dimensional tolerance of zero (0). For this reason, when the distal end bearing 9 is inserted, it is necessary to eliminate the misalignment (i.e. to perform alignment) based on the dimensional tolerance.
As other conventional documents related to the present invention, there is Patent Document 3 in addition to Patent Documents 1 and 2. Patent Document 3 discloses an invention relating to a rolling bearing insertion apparatus. However, in the case of the invention disclosed in Patent Document 3, a member (corresponding to the worm accommodation portion of the previous example) near a portion into which a rolling bearing is inserted needs to be provided with a centering shaft to be inserted into the rolling bearing. Therefore, the shape of the member disposed near the portion into which the rolling bearing is inserted is very limited. For example, in the structure in which the worm accommodation portion 14 integrally formed as illustrated in FIGS. 5, 6A, 6B, and 6C, the centering shaft cannot be provided. Therefore, such a structure cannot be adopted.